1. Field of the invention
The present invention concerns a method and a device for manufacturing a cathode ray tube. It concerns more particularly the heating of electrodes of electrostatic lenses of the electron gun, this heating being intended to eliminate the gaseous particles of the bulb under vacuum.
2. Description of the prior art
It is known that a cathode ray tube, especially a color television tube, comprises at least one electron gun in order to stimulate the cathodoluminescent materials (phosphors) on the screen. Said electron gun(s) is(are) disposed to the rear of the tube in a narrowed cylindrical part.
Such an electron gun comprises an emitting cathode heated by a filament and an assembly of electrodes, generally called grids, certain of which constitute electrostatic lenses. This assembly comprises an electrode or Wehnelt cylinder G.sub.1 raised to a variable voltage, usually negative with respect to the cathode, to order the extraction of the electrons, especially their quantity, a second electrode G.sub.2 for accelerating the electrons beams and two electrostatic lenses for concentrating the beam.
The invention concerns the case where each lens is of the bi-potential type, the first of said lenses comprising two electrodes G.sub.3 and G.sub.4, one of which G.sub.3, which immediately follows (towards the screen) the electrode G.sub.2, has an elongated form and is raised to a potential of about +7 kV and the other G.sub.4, which has a relatively flattened form in the example, and is raised to a potential of about +25 kV. The second lens for concentrating the beam is formed of two electrodes having an elongated form of which the first G.sub.5, which immediately follows the electrode G.sub.4 is brought to the same potential as the first electrode G.sub.3 of the first lens and of which the second lens G.sub.6 is raised to the same potential, of about +25 kV, as electrode G.sub.4 of the first lens.
During manufacture of the tube, after installation of the gun and the putting under vacuum of the bulb, the cathode(s) is(are) heated to a normal operating temperature in order to shape and stabilize the material forming this cathode and thereafter voltages are applied to the electrodes in order to heat said electrodes and thereby clean the tube, through elimination of the undesirable gaseous particles, especially the particles included in the metal forming the electrodes, all these particles being absorbed by a capsule called a getter.
It is important that this cleaning be carried out thoroughly since, in the event of incomplete cleaning, during normal working of the tube an after-glow phenomenon appears in the form of a luminous emission during a considerable time after switching off the television receiving set (or other visualization device) fitted with the tube.